1. Field of the Invention
The present invention relates to an isolator apparatus for isolating a pressure sensor in an instrument such as a pressure transmitter from a fluid such as pressurized process fluid.
2. Description of the Prior Art
A process fluid monitored by an instrument can be corrosive and can damage a pressure sensor in the instrument if direct contact between the fluid and the sensor occurs. An isolator arrangement can be interposed between the pressure sensor and an input flange or a manifold which couples the instrument to a conduit carrying the fluid.
Such a prior art isolator arrangement comprises an isolation diaphragm, isolation fluid and a pressure sealing ring. A thin, flexible, substantially round isolation diaphragm is disposed in a cylindrical inlet port provided in the instrument's housing for receiving the process fluid and deflects in response to pressure exerted on a first side of the diaphragm by the process fluid. The diaphragm is sealingly joined about its outer border, such as by a continuous weld, to a substantially cylindrical side wall of the inlet port thereby sealing the outer border of the diaphragm to the port. A central deflectable portion of the diaphragm is surrounded by the outer border and comprises the active region responsive to pressure. A second side of the diaphragm opposite the process fluid is open to a sealed isolation chamber provided in the instrument, which is substantially filled with a substantially incompressible fluid, such as silicone oil. The isolation fluid couples pressure through a passageway provided in the instrument leading from the isolation chamber to the isolated pressure sensor. Movement of the isolation fluid in response to deflection of the isolation diaphragm thus transfers the process fluid's pressure to the isolated pressure sensor. The pressure sealing ring, such as an "O ring" type made of resilient material, is disposed around the isolation diaphragm and sealingly couples process fluid from the input flange to the isolation diaphragm. In such a prior art arrangement, the sealing ring is compressed between the input flange and the isolation diaphragm. This compression prevents deflection of a portion of the isolator diaphragm adjacent the sealing ring. The annular seal provided around the isolation diaphragm by the pressure sealing ring thus limits the effective area of the isolation diaphragm's active region.
Isolation fluids can expand as temperature increases. Thus, as increased operating temperature can cause an expansion of the isolation fluid which is confined within the enclosed isolation chamber and passageway of the instrument, the isolation diaphragm's active region will respond in the effective area by deflecting to accommodate the increased isolation fluid volume. The accommodation of the expanded isolation fluid volume increases stress in the diaphragm. The stressed diaphragm exerts a pressure on the enclosed isolation fluid and that pressure is sensed by the sensor. The sensor senses the pressure of the process fluid plus the pressure due to the isolator diaphragm stress. The sensed pressure thus suffers from temperature-induced errors. These pressure measurement errors are generally proportional to the change in isolation fluid pressure divided by the change in volume due to diaphragm movement (dp/dv). Since dp/dv is an isolation diaphragm parameter which is a strong function of the effective area of the diaphragm's active region, it is desirable to use an isolation diaphragm having a large effective area, such that it is sufficiently compliant to accommodate the volumetric changes in isolation fluid and thereby minimize pressure measurement error. Increasing, for example, the effective area of a round isolation diaphragm, by increasing the diameter of its deflectable effective area from 1 inch to 1.2 inches, can reduce temperature-induced pressure measurement errors by a factor of two or more.
In such prior art isolation arrangements, however, since the effective area of the active region can be no greater than the region surrounded by the pressure sealing ring, the sealing ring's size becomes a limiting factor. For example, a compressible sealing ring mounted within an inlet port over a round isolation diaphragm having a 1.25 inch diameter deflectable active region yields an effective area of the active region having a diameter of approximately only 1.0 inch when compressed between the input flange and diaphragm. Further diaphragm constraints arise from the use of increasingly smaller solid state pressure sensors with correspondingly reduced instrument housings, which further restrict the space available for isolation diaphragms. In pressure transmitters, the size of the sealing ring can be further restricted by the space available between bolts joining the transmitter to an industry standard flange adapter union.